Reaction between stainless steel and calcium citrate/other minerals

[DVNick]

I work for a vitamin manufacturer in the US. We recently manufactured a calcium citrate and magnesium oxide blend. While manufacturing on our Bosch encapsulation machines (not cheap) we noticed black markings within the capsules. We also noticed some black markings on the outside of the capsule, most likely happening during the polishing process in our stainless steel polisher. I'm under the impression that there is a reaction happening between the calcium and the stainless steel. I've searched and searched but have found no specific reaction that would cause this. I've referenced my OChem book with no results. I've asked some colleagues and no one knows. I know it is common problem in the pharma/nutraceutical industries, but i cant find any answers as to why. Can someone please explain to me what is happening between the stainless steel (316) and the calcium citrate and magnesium oxide product? I've seen some guesses that it's oxidation of the metal facilitated by the calcium but i'm having a hard time believing that. I thought it might be some sort of reaction with pressure, but the polishing marking would negate that.

 

Thanks in advance!

[Enthalpy]

Citric acid is known to corrode 304 steel, but I expected 316L to withstand.

What you need is a "compatibility list" or "compatibility table" for "Aisi 316". They are huge, you may find citrate and calcium in them.

Beware 316 and 316L have different corrosion resistance, especially at the weld seams.

[DVNick]

Thank you for your response. So would corrosion of the SS be transferring to the powder, creating a black marking? This metal has no weld seams.

 

Citric acid is known to corrode 304 steel, but I expected 316L to withstand.

What you need is a "compatibility list" or "compatibility table" for "Aisi 316". They are huge, you may find citrate and calcium in them.

Beware 316 and 316L have different corrosion resistance, especially at the weld seams.